Geological aggregate deposits, or simply aggregates, are naturally occurring materials, considered in bulk as mixtures of particles of varying sizes. Aggregates can include materials such as boulders, cobbles, gravel, sand, silt and other granular materials having little or no plasticity. Aggregates, are commonly used to manufacture concrete and to construct roads, railroads and drainage works.
Aggregate deposits may be partially or wholly exposed at the land surface. Frequently however, they are found under layers of overburden. Proximity to point-of-use, i.e. the construction site, is often a prime factor in exploiting an aggregate deposit. Another factor in exploiting an aggregate deposit is the economic cost of removing the overburden. The economics of removing the overburden is a function of the value of the aggregate to be exposed and may depend on whether the aggregate deposit contains other minerals. These minerals may be metallic, non-metallic and/or precious minerals, depending on the geological origin of the aggregate deposit.
Over the years, various techniques have been developed for aggregate deposit prospecting. There are two principal techniques currently in use. The first technique relies on local knowledge of deposits which has been revealed by events such as surface exposure, road cuts, water wells and surface excavation. The other technique encompasses surficial geological investigations including the interpretation of aerial photographs. Surficial geological investigations involve identifying areas which have a high potential for containing aggregates based on the surface expression of geological phenomena that could have created the deposits (e.g. glacial processes). Potential sites for aggregates are then tested using on-site evaluation methods, including visual examination and electromagnetic surveys; and sub-surface methods such as drilling or excavation. The purpose of the on-site evaluation is to confirm and delineate the deposit, while also estimating the quality and composition of the material in the deposit, in addition to determining overburden thickness, depth to water table etc.
The known methods of aggregate exploration have enjoyed variable success in predicting aggregate deposit sites. Aerial photograph interpretation relies heavily on the surface expression of sub-terranean aggregate deposits or related geological events. As a result, its effectiveness is limited where the surface expression of the buried aggregate deposit is masked by subsequent deposits, erosion, plant growth or agricultural disturbances. Other remote sensing techniques, such as airborne electromagnetic and potential field sensing and satellite imaging, have been applied, but with limited success in locating commercially viable aggregate deposits.
It is estimated by the prospecting community that the success rate for aggregate prospecting is less than 5% using the above conventional methods. In other words, on average, less than 5% of the aggregate deposits identified using conventional techniques have yielded commercially viable sites.
According to the present invention, it has been found that aggregate deposits at or near the earth's surface are associated with a unique radioelement signature.
This radioelement signature, generally unique to aggregate deposits, can be detected using Gamma Ray Spectrometry (GRS) methods. Gamma Ray Spectrometry involves measuring sub-surface, near-surface and surface emanations of naturally occurring gamma radiation. For years, GRS has been used as an aid in prospecting for certain mineral deposits. Initial applications of GRS included detecting high magnitude anomalies in the naturally occurring gamma radiation caused by outcrops and other features containing enrichments of uranium and thorium minerals. More recently, GRS techniques have been applied to locating sub-terranean petroleum-bearing deposits as disclosed by Foote in U.S. Pat. No. 3,919,547.
In summary, it is an object of the present invention to provide a reliable method, incorporating GRS methods, for identifying and locating aggregate deposits and clay-rich soils at or near the earth's surface. The method, according to the present invention, achieves an improvement in the likelihood of success over known methods (which are successful only approximately 1 out of 20 times) for locating commercial deposits. It will be appreciated that the method according to the present invention can provide a breakthrough for aggregate deposit prospecting.